3.3 Volt trouble

Discussion in 'DM800SE-E Hardware troubles and Repair support.' started by rtznfrtz, Sep 4, 2016.

  1. rtznfrtz

    rtznfrtz New Member

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    HI
    found i have this layout :)
    my 3.3 V supply is a bit of a mess. "usually" i get 1.67 V at the pins of U1102 and U1000. of course the box is not booting. but when i fiddle around witj my multimeter it sometimes suddenly switches to 3.3V and box starts booting.
    Any mor info or data availlable - so i can check?
    thx for your help
     
  2. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Hi,

    I'm not sure why you're talking about a U1000, but assume that you mean the U1100.
    But also has these less to do with the 3.3v problem.

    But about the issue the U1102, 3.3v dropping to 1.67v.
    Because you say that it sometimes gives the correct voltage, then I have no doubts that the Dc ic seems to be good.
    It seems to me to come from other causes such as ..

    It basically could happen if one of the components around this Dc to Dc converter have a bad connection to one of the resistors or ceramic caps.
    For example, the pin 1, vbst must have a connection to the c1123, and the other side from the c1123 must have a connection to the coil, and the pin 3 SW.
    The other side of the coil is the output to the mainboard, but the connection to the feetback pin 5 (vfb) must also be healthy.
    This line has a resistor between it, with the marking code 24C on it.

    In any case, you need to check all the connections between the parts around this U1102, if one of the resistors, or caps has a bad connection, then the voltage will drop to 1.x volts.

    Check the connections between them, so orange to orange, yellow to yellow etc..
    Note that Red is not so important, it's the 12v input, however the 12v to the resistor which goes to the EN pin must also be present, otherwise the voltage drops also.
    u1102.JPG

    Other possibilities are..
    Or tip with a soldering iron on the soldering of the components.
    Perhaps one of the soldering is poor.

    Another possibility is also a bad coil L1112, or a bad cap C1128.
     
  3. rtznfrtz

    rtznfrtz New Member

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    Thx
    measured them all - no bad or missing contact or shorted circuit
    the U1000 i mentioned because it has the same voltage at pin 2 and 3

    PS: the green marker next to R1170 has no connection to the green marker at R116 - but i think this is a bad reference because R1170 is part of 5V circuit
     
  4. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Yeah indeed, that is wrong, for the U1102 it needs to be the Q1107/R1121/R1123 on the print side.